An optical memory technology using an optical disk having a pit-shaped pattern as a high-density, large-volume storage medium has been put to practical use with its application range being expanded to a digital audio disk, video disk, document file disk, data file, and so on. The function of successfully recording/reproducing information onto/from an optical disk through a finely narrowed light beam with a high degree of reliability can be roughly divided into a condensing function for forming a micro spot narrowed down to a diffraction limit, optical system focus control (focus servo), tracking control and pit signal (information signal) detection.
In recent years, with the advance in optical system design technologies and shortened wavelengths of a semiconductor laser serving as a light source, an optical disk having a higher-density storage capacity than a conventional one is being developed. As an approach to realizing higher-density storage capacities, a method of increasing numerical aperture (NA) at the side of the optical disk in a condensing optical system which condenses a light beam on the optical disk to a micro level is under study. The problem in that case is an increase in the amount of aberration due to an inclination (so-called tilt) of an optical axis. Increasing NA increases the amount of aberration which occurs due to tilt. To prevent this, the thickness of the substrate (base material) of the optical disk may be reduced.
A compact disk (CD) which can be said to be the first generation optical disk uses infrared light (wavelength λ3: 780 nm to 820 nm), an objective lens having an NA of 0.45 and has a disk base material of 1.2 mm in thickness. The DVD, the second generation, uses red light (wavelength λ2: 630 nm to 680 nm, standard wavelength 650 nm), an objective lens having an NA of 0.6 and a disk base material of 0.6 mm in thickness. The optical disk, the third generation, uses blue light (wavelength λ1: 390 nm to 415 nm, standard wavelength 405 nm), an objective lens having an NA of 0.85 and a disk base material of 0.1 mm in thickness.
In the present specification, the base material thickness refers to a distance from the plane onto which a light beam impinges in the optical disk used as an information recording medium to the information recording surface of the optical disk.
Furthermore, for the purpose of realizing a larger-capacity optical disk, a disk having a multi-layer structure with two or more recording layers is available on the market or under study.
When focus control is applied to such a disk having a multi-layer structure, or focus control is initially applied from a state in which focus servo is not functioning yet, that is, beam spot positioning is performed, it is important to focus on a desired recording layer in which data such as various disk characteristics is written and position the beam spot, also for the purpose of shortening a wait time until an operation is started.
A conventionally proposed beam spot positioning method for a two-layer optical disk will be explained below.
FIG. 10 is an optical information apparatus which records/reproduces data onto/from a two-layer optical disk according to a conventional technology. In FIG. 10, a two-layer optical disk 109 is placed on a turn table 182 and rotated by a motor 164 as a rotation system. An optical head apparatus 155 is roughly moved to a track in which desired information of the two-layer optical disk 109 exists by a driving apparatus 151 of the optical head apparatus.
The optical head apparatus 155 also sends a focus error signal or tracking error signal to an electric circuit 153 in accordance with the positional relationship with the two-layer optical disk 109. In response to this signal, the electric circuit 153 sends a signal for inching an objective lens to the optical head apparatus 155. According to this signal, the optical head apparatus 155 carries out focus control or tracking control on the two-layer optical disk 109 and the optical head apparatus 155 reads, writes or erases information.
FIG. 11 is a flow chart showing a beam spot positioning method for the conventional two-layer optical disk, FIG. 12 illustrates a focus error signal waveform and FIG. 13 illustrates a positional relationship between the optical disk and objective lens during beam spot positioning for the conventional two-layer optical disk. In FIG. 13, reference numeral 120 denotes a two-layer optical disk whose information recording layer has a two-layer structure made up of a first layer 120b and a second layer 120c and 130 denotes an objective lens. Reference numeral 170 denotes a focus driving apparatus which drives the objective lens 130 in a direction perpendicular to a principal plane including a surface 120a of the two-layer optical disk 120 and corresponds to the driving apparatus 151 in FIG. 10. Furthermore, as shown in FIG. 12, the focus error signal is a signal whose voltage fluctuates in a positive or negative direction in the vicinity of the recording surface with respect to a predetermined reference voltage E according to the distance from the objective lens 130 and two-layer optical disk 120.
Hereinafter, a case of reproduction of information will be explained as an example according to the flow chart in FIG. 11. When a reproduction command on the two-layer optical disk 120 is issued (S101), a laser diode (not shown) is caused to emit light (S102), then the focus driving apparatus 170 is driven (S103) and the objective lens 130 is moved within a predetermined movement range. The electric circuit 153 turns ON the focus servo (S104) and monitors the focus error signal of the first layer shown by a waveform A in FIG. 12 when the objective lens 130 is moving. When it is detected that the objective lens has reached point B in FIG. 11 which is an in-focus point of the first layer 120b (S105), the focus servo is started (S106) using this focus error signal of the first layer 120b as a control signal, a focus jump is made to point D in FIG. 11 which is the position of the in-focus point of the first layer 120c (S107) (this operation is carried out as a movement of the objective lens 130 from a state in which the beam spot is positioned at the first layer 120b shown in FIG. 13(b) to a state in which the beam spot is positioned at the second layer 120c shown in FIG. 13(c)), the focus servo is started (S108) using the focus error signal of the second layer 120c shown by a waveform C in FIG. 12 as a control signal and a data read of the second layer is carried out (S109).
According to the beam spot positioning method for the above described two-layer optical disk, when a data read is performed from the second layer 120c, the focus servo of the first layer 120b is started first, and then a focus jump is made to start the focus servo for the second layer 120c. For this reason, a time is required until a data read of the second layer.
Thus, a beam spot positioning method intended to make data access in a shorter time using a drive apparatus which records/reproduces data onto/from a two-layer optical disk is disclosed in Japanese Patent Laid-Open No. 9-161284. The structure of the optical information apparatus which performs beam spot positioning is the same as that of the conventional example shown in FIG. 10 and only the control operation is different, and so detailed explanations thereof will be omitted.
FIG. 14 is a flow chart showing a beam spot positioning method for a two-layer optical disk of the conventional example, FIG. 15 illustrates a focus error signal waveform and FIG. 13 illustrates the positional relationship between the two-layer optical disk 120 and objective lens 130 during beam spot positioning. Hereinafter, a case of reproduction of information will be explained as an example according to the flow chart in FIG. 14.
When a reproduction command for the two-layer optical disk 120 is issued (S201), the laser diode is caused to emit light (initial state shown in FIG. 13(a)) (S202). Then, the focus driving apparatus 170 moves the objective lens 130 in a direction perpendicular to the information recording surface of the two-layer optical disk 120 within a predetermined range of distance (S203). As the objective lens 130 moves, the electric circuit 153 starts to detect the focus error signal of the first layer 120b shown by a signal waveform A in FIG. 15 (S204), and detects a period G during which the voltage of the focus error signal is lower than a predetermined focus error signal detection slice level voltage F of the first layer 120b. 
Then, when a time point H at which the focus error signal voltage falls below the first layer focus error signal detection slice level voltage F is detected again, the focus servo is turned ON (S205).
Next, the focus error signal C of the second layer 120c indicated by a signal waveform C in FIG. 15 is monitored and if it is detected that the objective lens 130 has reached the position corresponding to an in-focus point D of the second layer 120c (S206), the focus servo is started using the second layer focus error signal C as a control signal (S208) and a data read of the second layer is performed (S209).
However, the above described conventional beam spot positioning method has the following problems. That is, as shown in FIG. 15, the focus error signals of the first layer 120b and second layer 120c are detected by detecting the waveforms A, B, but this detection is performed by detecting the voltage at a peak of the waveform or a position corresponding to predetermined displacement from a focus error signal reference voltage E. At this time, if, for example, the reflective index of the second layer 120c is low and the peak and voltage corresponding to the focus error signal of the second layer 120c cannot be detected, the objective lens 130 may continue to move in search of the focus error signal of the second layer, and collide with the optical disk 120, causing damage to the objective lens 130 or optical disk 120.
Furthermore, when the reflective index of the first layer 120b is low and the focus error signal of the first layer 120b cannot be detected for the same reason as that described above, the focus error signal of the second layer 120c may be mistaken for the focus error signal of the first layer and the objective lens 130 may continue to move in search of the (inexistent) focus error signal of the second layer 120c, and finally collide with the optical disk 120, causing damage to the objective lens 130 or optical disk 120.